Mixture Question #1

Question

You need a 25% alcohol solution. On hand, you have a 225 mL of a 75% alcohol mixture. How much pure water will you need to add to obtain the desired solution?

Answer 1

Hello!

Since you are starting with a 75% alcohol mixture with a total volume of 225 mL, you can figure out how much of starting mixture is alcohol and how much is water.

225 mL

75% alcohol

Which means the remaining 25% is water.

225 mL * (75/100) = 168.75 mL of alcohol

225 mL * (25/100) = 56.25 mL of water

So now you want to add water to change the mixture composition to 25% alcohol and the remaining 75% as water. You know your starting mixture has 168.75 mL of alcohol, which is 75% alcohol content. In order to get to 25% alcohol content, we have to dilute the starting mixture by x mL of pure water.

168.75 mL = (25/100) * (225 mL + x)

x = 450 mL of pure water

So you need to add 450 mL of pure water to the starting mixture to obtain the desired 25% alcohol solution.